CO Hydrogenation
The increasing demand for energy has led to renewed interest in non-traditional sources of feedstocks. For instance, coal was used as a source for gaseous fuel ("town gas") during earlier parts of this century. Much work was done at that time to produce higher molecular weight hydrocarbons from the synthesis gas. That work, involving carbon monoxide (CO) hydrogenation chemistry, was revived by Germany during World War II and currently enjoys moderate use at the SASOL plants in South Africa.
Research continues on the CO-H.sub.2 chemistry because of the potential for converting low value feedstocks into higher value products. Carbon dioxide (CO.sub.2) is the major product of combustion processes and is available at relatively high pressure as a diluent in many gas fields throughout the world. However, relatively little attention has been paid to the conversion of CO.sub.2 into hydrocarbons.
U.S. Pat. Nos. 4,544,671, Fiato et al, and 4,544,674, Soled et al, describe iron-carbide based catalyst used to convert a feed stream of CO and H.sub.2 to olefins. U.S. Pat. No. 4,279,830, Haag et al, is also directed to a process for converting CO and H.sub.2 to C.sub.3.sup.+ products. Haag et al discloses a procedure for generating an iron-carbide based catalyst. However, the results obtained in these three cases did not show a product rich in C.sub.2 -C.sub.4 olefins.
Frequently, CO.sub.2 is produced when iron based catalysts are used in CO hydrogenation. The CO.sub.2 is formed from the simultaneously occurring water gas shift reaction (WGS): EQU H.sub.2 O+CO.revreaction.H.sub.2 O+CO.sub.2 (WGS)
where the water is formed as a by-product of the CO hydrogenation. The CO.sub.2 formed in the WGS reaction can be suppressed by adding CO.sub.2 to the feed stream following Le Chataliens principle. For example, U.S. Pat. Nos. 2,585,981, 2,486,894 and 2,486,895 are directed to the catalytic hydrogenation of CO with an iron-containing catalyst to produce hydrocarbons, oxygenated hydrocarbons and mixtures thereof. There, CO.sub.2 was added to the reactant stream until the ratio of moles of hydrogen to moles of carbon dioxide and carbon monoxide is about 0.6. This causes the net production of CO.sub.2 in the reaction zone to be substantially inhibited. In addition, the amount of CO.sub.2 was regulated to result in minimal CO.sub.2 production, with care taken not to convert the CO.sub.2 to hydrocarbons.
U.K. Patent No. 709,560 is another example of CO hydrogenation where the presence of CO.sub.2 acts to suppress the net formation of CO.sub.2 produced by the water gas shift reaction. There is no disclosure or suggestion that CO.sub.2 hydrogenation is taking place.
In the past, CO.sub.2 was not looked to as a favorable component for the hydrogenation feed stream for a number of reasons. Carensbourg et al, Reviews in Inorganic Chemistry, Vol. 7, No. 4, p. 315-339 (1986), for instance, informs us that (i) hydrogenation of CO.sub.2 was generally much more selective toward methane than CO which nearly always produces a significant amount of higher hydrocarbons and (ii) that the activation energy for the methanation of CO.sub.2 was lower than that for CO, resulting in significant methane production from CO.sub.2 at temperatures where CO methanation did not occur and, finally, (iii) that certain catalysts were more efficient at methanating CO.sub.2 than CO, while the reverse was true for another group of catalysts.